This invention relates to a process of producing tartaric acid from a raw material whose dry matter comprises at least 5 wt-% potassium hydrogentartrate (KHT). As raw material, there may for instance be used wine yeast and/or tartar, which are obtained during the production of wine. Preferably, the KHT content of the raw material, based on dry matter, is at least 10 wt-%.
It is the object underlying the invention to produce a purified tartaric acid, which may also be useful for foodstuffs, in an environmentally beneficial and inexpensive way. In accordance with the invention this is achieved in
a) that in a first reaction stage the raw material is mixed with aqueous potassium hydroxide solution, and KHT is virtually completely reacted to form dipotassium tartrate (DKT),
b) that from the first reaction stage there is withdrawn a first aqueous solution containing DKT, impurities are removed, and in a precipitation stage the first solution is mixed with added acid at a pH value of 2.0 to 5.0, and a suspension containing crystallized KHT is produced,
c) that crystallized KHT is separated from the suspension, washed with water, and there is produced a second aqueous solution saturated with KHT for at least 80 wt-% and
d) that from the second aqueous solution potassium is removed, an aqueous tartaric acid solution is produced, and from the tartaric acid solution water is at least partly removed.
Tartaric acid has the empirical formula H6C4O6, potassium hydrogentartrate is KH5C4O6 and dipotassium tartrate is K2H4C4O4. In the first reaction stage a) KHT is reacted with KOH to form DKT (K2H4C4O6) plus water, where preferably a pH value of 7 to 8 is employed. DKT has a much better solubility in water than KHT.
The first aqueous solution containing DKT is mixed with added acid in the precipitation stage, where the poorly soluble KHT is formed. It is recommended to use tartaric acid as acid, when the introduction of foreign substances should rather be avoided. It is, however, very well possible to employ for instance H2SO4 or HCl instead of tartaric acid, and to remove the sulfate or chloride ions from the process at another point.